As wiring and electrical power distribution systems have become more sophisticated and as greater attention has been focused on the safety of personnel who must work on, in, and around such equipment, much ingenuity and inventive power has been applied to increase safety and minimize the possibility of any inadvertent electrical shock. A principal concept to assure safety is that of connecting all exposed metallic surfaces to ground potential which means quite literally, an electrical continuity to the earth. If all exposed metal surfaces are at gound potential, then there can be little, if any, electrical potential between any two surfaces and/or any surface and the earth and therefore, an individual touching any of these surfaces either singly or in combination, will not be subjected to a potential difference and the possibility of an electric shock.
This principle, in concept, is very sound. However, practicing the principle without any deviation, especially in the event of some fault condition, has not always been as simple and obvious as it might appear. An obvious possibility of danger is a discontinuity or interruption of the ground connection. To avoid this possibility, ground connections never pass through switches nor are any fuses or circuit breakers included in ground connections. Making a good connection to the earth is a study in itself and the techniques will vary depending upon the composition of the earth and the normal weather conditions. Since the techniques used are well known to those skilled in the appropriate arts and since such techniques do not have a direct bearing on the structure to be described, no further mention will be made thereof.
One of the more obvious ways in which a ground connection may be inadvertently disconnected from an exposed metallic surface will occur when the ground connector is called upon to carry a larger current than its physical characteristics and/or the quality of its electrical connection will allow. In such a situation, the ground conductor or connector may melt or vaporize, and interrupt the ground connection thereby leaving exposed metal surfaces at a dangerous potential. To avoid this possibility, it is obvious that the ground conductor and connectors must be capable of carrying a current at least as large as any which they might be called upon to carry in the event of the worst possible fault condition that can be conceived.
An exposed metal surface may not be at ground potential even when the ground connector is connected thereto if there is any substantial resistance between the exposed metal surface and the ultimate ground connection. Excessive resistance in the ground connection may result from faulty equipment design and/or faulty installation or connection. It may also result from part of the ground connection being required to pass through members which do not have a sufficiently low resistance and/or an adequate cross sectional area of the conducting member.
U.S. Pat. No. 3,365,693 issued Jan. 23, 1968 to F. L. Browne discloses a grounding lug suitable for connecting to a conduit fitting and the like.
U.S. Pat. No. 3,706,959 issued Dec. 19, 1972 to A. R. Norden discloses a ground connector for a conduit.
U.S. Pat. No. 4,248,490 issued Feb. 3, 1982 to W. W. Bachle discloses a lay-in lug having a conduction pad for reducing the resistance between the ground conductor and the bushing to be grounded by the grounding conductor.
U.S. Pat. No. 4,320,882 issued May 23, 1982 to W. W. Bachle discloses a ground clamp for providing increased conductivity between a conduit member and a cable tray or structural member. Each of these patents is assigned to the same assignee as the present invention.